A. Field of the Invention
The present invention relates to equipment for testing electrical circuits, more particularly, the invention relates to a portable, hand-held instrument particularly adapted to performing voltage and continuity tests on electrical systems of autos, trucks and similar vehicles.
B. Description of Background Art
Maintenance of the large number of automobiles, trucks and other vehicles on the road has become an increasingly technologically sophisticated task. The requirement for sophisticated test equipment is a result of the increasing complexity of modern vehicles. That complexity in turn has been necessitated by the effort to make vehicles more energy efficient, less polluting, and safer.
Because of the complexity of modern vehicles, a wide variety of sophisticated test equipment has been made available for troubleshooting and adjusting the various systems of vehicles. Much of the test equipment used for vehicle maintenance employs electronic circuitry, which can range from the relatively simple to substantially complex. As might be expected, more complex electronic test equipment tends to be expensive, fragile, and relatively large.
In spite of the complexity of modern vehicle systems, and the test equipment used to service them, much initial trouble shooting performed on the road and, to a lesser extent, at service facilities, often includes relatively simple electrical tests. Such tests typically include resistance and voltage measurements and are often made by use of a volt-ohm-ammeter, or multi-meter. However, it is sometimes desirable to perform even simpler electrical diagnostic tests on vehicle systems, such as the presence of voltages, and electrical continuity. Thus, a variety of devices have been designed to perform simple voltage detection and/or continuity measurements for vehicle systems, telephone wiring and the like. The following United States patents related to such electrical test equipment are known to the present inventor.
Garrett. U.S. Pat. No. 3,600,678, Aug. 17, 1971, Solid-State Polarity Tester For Telephone Equipment:
Discloses a solid state polarity tester provided with ground and battery terminals for connection to like contacts of a battery in a telephone central office switchroom, and with a probe to test polarity of telephone equipment, the same embodying two similar circuits between the probe and each respective terminal, each said circuit including an indicator lamp and a transistor that switches on to close the circuit through one lamp according to the direction of current flow, and shorting out the other lamp. The tester includes a third circuit between the probe and the ground terminal and including a glow tube energized to indicate either high-level DC or AC is in contact with the probe.
Naylor, U.S. Pat. No. 3,619,775, Nov. 9, 1971, Polarity And Voltage Level Detecting Test Probe:
Discloses an electrical test instrument for use in servicing digital computers which provides a digital output by means of two lamps indicating the characteristics of a voltage level or pulse or train of pulses at a point in the computer. The instrument comprises a test probe connected through a selectively inserted Zener diode to a pair of cascaded amplifiers each of which drives a gating circuit and a control circuit for providing a conditioning signal to the gates. The gate circuits in turn control the indicating lamps. A pair of flip-flops may be selectively placed in the circuit between the amplifiers and the gates when the test instrument is used to indicate statically the condition of the first pulse received by the probe.
Hall, U.S. Pat. No. 3,806,803, Apr. 23. 1974, Portable Voltage Sampling Probe Device:
Discloses a portable solid state probe device used to sample the voltage for polarity at various points in either telephone central office equipment or outside key equipment. The device itself includes first and second light emitting diodes which will respectively become energized when the sampled voltage is of positive or negative polarity. The device includes Zener regulating diodes to provide a substantially constant voltage to the light emitting diodes so that the device will function properly when supplied with either 48 volts as is available in central office equipment or 24 volts as is only available in field key equipment.
Stewart, U.S. Pat. No. 4,027,236, May 31, 1977, Voltage And Continuity Checker:
Discloses a portable test instrument for testing electrical continuity, and diode polarity, AC and DC voltage amplitudes and DC voltage polarity. An electronic switch device has main electrodes connected in an indicating circuit with an electrical supply, a light-emitting diode and a current-limiting resistor. A double pole, double throw switch is manually switchable between continuity and voltage test positions, and with four resistors defines an input circuit interposed between the control electrode of the electronic switch device and probes connectible to a circuit or the like to be tested. In both the voltage and continuity test modes of the instrument, the light-emitting diode is energized only by current from the electrical supply within the indicating circuit. Though proper circuit operation is obtained by manual manipulation of the mode switch between its voltage and continuity positions, the light-emitting diode is automatically protected from damage should the test probes engage a voltage circuit (within the operating voltage range of the instrument) while the instrument is in continuity mode.
Bloxam, U.S. Pat. No. 4,028,621, Jun. 7, 1977, Portable Automivity Test Probe:
Discloses an improved electronic DC test probe particularly suitable for rapidly and flexibly trouble-shooting automotive electrical systems. The test probe features highly reliable electronic circuits energized from an automotive battery through a pair of connector members suitable for connection to the terminals of the battery. A conductive test prod and a pair of indicator lamps are housed within a hand-held probe housing and enable an operator to rapidly trouble shoot the entire electrical system without alteration of any kind to the battery terminal connections. Electronic reference voltage establishing circuits protect the probe circuits and extend the operative life of the indicator lamps. The normally extinguished indicator lamps are illuminated only during actual test measurements with an intensity that varies proportionately with the measured signal. In one embodiment of the invention a single conductor extends from the probe chassis to a common terminal adjacent the connector members at the battery terminals. Another embodiment includes twin-beam lamps within the probe housing, enabling the probe to double as a trouble light.
Ellis, U.S. Pat. No. 4,366,434, Dec. 28, 1982, Voltage Detectors And Electrical Continuity Checkers:
Discloses a combined voltage detector and electrical continuity checker in the form of a pencil having a needle capable of piercing electrical insulation, a pair of light emitting diodes for determining the polarity of the voltage, a third light emitting diode and associated circuitry to perform a continuity check, a flexible cable and chip which can be plugged into the pencil to perform either one of the tests in conjunction with the needle, and biasing arrangements for the light emitting diodes in order to lower their threshold of conduction.
Nolan, U.S. Pat. No. 4,540,940, Sept. 10, 1985, Circuit Tester For Automotive Electrical Systems:
Discloses a circuit tester for automotive electrical systems comprising a body member of "pistol-grip" configuration having an electrical socket at one end thereof and an electrically conductive probe extending from the other end thereof. An electrical lead is provided for connecting the terminals of the automobile battery to the socket. The lead has a plug at one end thereof which is reversibly receivable by the socket to enable the tester to be placed in either a positive test mode or a negative test mode. A first electrical lead member extends through the body member from the socket to the probe and has a light imposed therein. A second electrical lead extends from the socket to the probe but has a normally open, manually operated switch imposed therein. The second electrical lead member also has a replaceable fuse assembly imposed therein. The light will be illuminated when the switch is closed whether or not the probe is in electrical contact with the electrical system of the automobile or a component thereof. The light will be illuminated when the switch is opened and the probe is in electrical contact with the automobile electrical system or a component thereof when electrical continuity exists. When the switch is closed, electrical current is supplied to the probe.
Cestaro, U.S. Pat. No. 4,599,557, Jul. 8, 1986, Logic Probe:
Discloses a test probe for determining the logic state of a digital logic circuit, which probe utilizes threshold detectors to determine the relative voltage level of a test point, and gate circuits to enable various voltage conditions, e.g., a ground, high, low, uncommitted, or a supply voltage, or a random pulse or a pulse train, to be monitored and the test result indicated on an LED alpha display device. The threshold detectors are selectively settable so that the probe is particularly adapted to test TTL and CMOS circuitry.
Stephens, U.S. Pat. No. 4,999,574, Mar. 12, 1991, Switchable Automotive Circuit Test Light:
Discloses an automotive circuit test light powered by an external power source such as a vehicle battery, and a first light emitting diode for indicating that the device has been connected to a suitable power source. A probe protruding from a housing for the device is connected to a normally closed contact of a first pole of a double-pole, double throw switch and to a normally open contact of a second pole of the switch. The other two transfer contacts of the switch are connected across an external power source. A second LED and series current limiting resistor are connected in series with the first and second poles of the switch. Thus, with the switch in a first position, the second LED will light only when the probe contacts a point at a negative potential with respect to positive terminal of the external voltage source. With the switch in this opposite position, the second LED will light only when the probe contacts a point at a positive potential with respect to the negative terminal of the external voltage source.
The Bloxam patent discloses a pair of lamps connected in series with a test probe. One of the lamps is connected to the cathode of a first diode, while the other is connected to the anode of a second diode. The other terminals of the two diodes are connected to the input terminal of half-wave rectifier, the output terminals of which are connected to the plus and minus terminals of a battery. Thus, the Bloxam device functions as a polarity-sensitive voltage tester.
The Ellis patent discloses inverse parallel LED's and separate D.C. bias voltage sources to test voltage polarity and continuity.
The Stephens patent discloses a automotive circuit tester having a first LED power-on indicator, and a second LED switchable by means of DPDT switch to place the second LED in series with a test probe and either the minus or plus terminal of an external voltage source, thereby providing a polarity indicating voltage tester.
None of the patents described above use a center-off SPDT switch to provide polarity indicating voltage sensing, and polarity selectable continuity testing, and the present inventor is unaware of any other prior art devices having those features. Also, no prior art known to the present inventor discloses or suggests the use of a polarity selectable probe voltage for continuity testing in conjunction with a circuit breaker to permit momentarily contacting a low impedance source or ground to a test point, to determine whether the point is truly at ground or +V, respectively, rather than being connected thereto through a low impedance.
The present invention was conceived of to provide an improved hand-held portable electrical instrument for electrical testing.